For circuit safety, a conventional audio-frequency amplifier uses a transformer to reduce utility power voltage firstly, then uses a rectifier to rectify mentioned power into DC voltage, and transforms DC voltage to an amplifying circuit as a power source eventually. However, the conversion efficiency of normal small-scaled transformer is lower than 85% to cause the operation efficiency of the audio-frequency amplifier to be poor. In addition, although the audio-frequency amplifier applies small-scaled transformer, bulk and weight of the conventional audio-frequency amplifier are difficult to reduce effectively because the bulk and weight of the transformer and attached heat-dissipation apparatus account for the most portions compared with whole the audio-frequency amplifier. Otherwise, the operation power of the amplifying circuit is constrained by the voltage switching of the transformer thus making the conventional audio-frequency amplifier with high power (250 W) being difficult to achieve.